Laptop Battery Replacement?

That behavior actually makes sense given how you’re using it.
Keeping the XPS plugged in at 100% almost all the time does accelerate battery wear, especially over 2+ years. Lithium-ion batteries age fastest when they’re:
Held at high charge (near 100%)
Kept warm (external monitors + constant power = more heat)
Rarely allowed to cycle
Locking the laptop overnight is fine, and BitLocker isn’t related, but when your UPS cuts power, the laptop suddenly has to rely on a battery that’s already degraded and sitting at a low effective capacity. That’s why it either drops to very low battery quickly or shuts off entirely.
A few points worth checking/doing:
In Dell BIOS / Dell Power Manager, set a charge limit (e.g. max 80%). This helps a lot going forward.
Run a battery health check (powercfg /batteryreport in Windows or Dell diagnostics) — it will likely show significant wear.
If you keep using a UPS mainly as a surge protector, consider replacing the UPS battery so it doesn’t randomly cut power.
At 2.5 years with this usage pattern, needing a battery replacement is normal, not a defect.
In short: nothing unusual or BitLocker-related here — it’s just a worn battery finally being exposed when AC power drops.